In general, each of organic EL devices using an organic material and constituting a display panel has a structure in that an anode as a transparent electrode, plural organic material layers including an organic light emitting layer and a cathode comprising a metal electrode are successively layered each as a thin film on a glass substrate as a display surface. In addition to the organic light emitting layer, the organic material layer includes a layer comprising a material having a hole transportability such as a hole injection layer and a hole transport layer, and a layer comprising a material having electron transportability such as an electron transport layer and an electron injection layer, and an organic EL device of a constitution provided with them has also been proposed. The electron injection layer also contains an in organic compound.
When an electric field is applied to an organic EL device of a stacked body of an organic light emitting layer and an electron or hole transport layer, holes are injected from the anode and electrons are injected from the cathode. The organic EL device utilizes emission of light that is emitted when the electrons and the holes are recombined in the organic light emitting layer to form exciters and they are returned to the ground state. For making the luminous efficiency higher and stably driving the device, a dye is sometimes doped as a guest material to the light emitting layer.
In recent years, in addition to the fluorescence material, use of a phosphorescence material for the light emitting layer has also been proposed, since it is considered that the probability for the occurrence of singlet exciters and triplet exciters after recombination of electrons and holes in the light emitting layer of the organic EL device is 1:3 and it is considered that a device also utilizing phosphorescence caused by the triplet exciters can attain the luminous efficiency three to four times as high as the device using fluorescence caused by the singlet exciters.
On the other hand, for reducing the power consumption improvement of the luminous efficiency and improvement of the driving stability of the organic EL device, it has been proposed to provide a hole blocking layer between the organic light emitting layer and the cathode for restricting the movement of the holes from the organic light emitting layer. By efficient accumulation of the holes in the light emitting layer by the hole blocking layer, the recombination probability with electrons can be improved to attain higher luminous efficiency. As the hole blocking material, it has been reported that phenanthroline derivatives and triazole derivatives are effective (refer to JP-A No. 8-109373 and JP-A No. 10-233284).
In existent organic EL devices utilizing phosphorescence emission, hole transporting materials capable of transporting holes have been used for the light emitting layer host material and materials having higher ionizing potential energy (Ip) than the post material of the light emitting layer such as phenanthroline derivatives, for example, 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline, i.e., BCP or aluminum chelate complex, for example, ((1,1′-biphenyl)-4-olato)bis(2-methyl-8-quinolinolato NI, 08) aluminum, i.e., BAlq are used as the hole blocking layer in the layer adjacent to the cathode side of the light emitting layer. In a case of using BCP for the hole blocking layer, while the emission characteristics at the initial stage are favorable, it involves a drawback that the driving life is extremely short. At present, materials having sufficiently high Ip and excellent in durability have not been known.
BAlq is excellent in durability but has a drawback of poor hole blocking ability since Ip is not sufficiently high. Accordingly, in a case of using BAlq as the hole blocking layer and tris(8-hydroxyquinolateo N1, 08) aluminum, i.e., Alq3 as the electron transport layer, the electron transport layer emits light. In the organic electroluminescence device utilizing red phosphorescence emission, the emission of Alq3 (green) results in deterioration of chromaticity (orange instead of red).
While it is effective to provide a light emitting layer of an organic phosphorescence material and a hole blocking layer for increasing the luminous efficiency of the organic EL device, it is necessary to further prolong the life of the device. It has been demanded for an organic EL device of high luminance efficiency that is continuously driven to emit light at high luminance with small quantity of current.